collect-license-metadata: Print a diff of the expected output

Cargo.lock

@@ -686,6 +686,7 @@ dependencies = [
  "anyhow",
  "serde",
  "serde_json",
+ "similar",
  "spdx-rs",
 ]
 

src/tools/collect-license-metadata/Cargo.toml

@@ -9,4 +9,5 @@ license = "MIT OR Apache-2.0"
 anyhow = "1.0.65"
 serde = { version = "1.0.147", features = ["derive"] }
 serde_json = "1.0.85"
+similar = "2.7.0"
 spdx-rs = "0.5.1"

src/tools/collect-license-metadata/src/main.rs

@@ -5,9 +5,21 @@ mod reuse;
 use std::path::PathBuf;
 
 use anyhow::{Context, Error};
+use similar::{ChangeTag, TextDiff};
 
 use crate::licenses::LicensesInterner;
 
+fn diff_text(expected: &str, actual: &str) {
+    for change in TextDiff::from_lines(expected, actual).iter_all_changes() {
+        let sign = match change.tag() {
+            ChangeTag::Delete => "-",
+            ChangeTag::Insert => "+",
+            ChangeTag::Equal => " ",
+        };
+        print!("{}{}", sign, change);
+    }
+}
+
 /// The entry point to the binary.
 ///
 /// You should probably let `bootstrap` execute this program instead of running it directly.
@@ -41,6 +53,8 @@ fn main() -> Result<(), Error> {
         if existing_json != output {
             eprintln!("The existing {} file is out of date.", dest.display());
             eprintln!("Run ./x run collect-license-metadata to update it.");
+            eprintln!("Diff:");
+            diff_text(&existing, &serde_json::to_string_pretty(&output).unwrap());
             anyhow::bail!("The existing {} file doesn't match what REUSE reports.", dest.display());
         }
         println!("license information matches");